Intrusion alarm systems are known for detecting the presence of an intruder in a zone under protection. In such systems, there is typically provided a central control unit and a plurality of remote transceivers each located in an area to be protected, the transceivers being interconnected by one or more multiwire cables to the control unit. The transceivers each include a transmitting and a receiving transducer and associated circuitry for propagating an energy pattern into the protected zone and for receiving signals returned therefrom. The central control unit includes signal processing circuitry for detection of intruder presence and for discriminating signals representative of intruder presence from spurious signals and noise. The cables interconnecting the transceivers with the control unit typically include a pair of lines carrying a balanced transmitter signal for the transmitting transducers, a shielded receiver line for conveying signals provided by the receiving transducers to the control unit for subsequent processing, and one or more tamper lines. The tamper line is usually connected in a closed loop from a transceiver to the control unit such that cutting of the tamper line is detectable as an alarm condition. Tampering with the lines other than the tamper line of the cable will not usually result in an alarm condition. In order to further enhance the security of an alarm system and particularly the interconnecting cables thereof, it would be preferable to monitor all wires of the system cabling to detect any tampering or changed signal conditions on any of the cable lines.